One of the largest contributors to the development of a hostile environment inside an aircraft cabin during a fire is the production of flammable and toxic vapors from soft fabrics and furnishings, the bulk of which are contained in the aircraft's seats. The flammable vapors produced by thermal decomposition of conventional foam cushions are considered to be the largest single factor contributing to this hostility factor during such a fire.
Among existing commercially used cushioning polymers, there is probably no better material from mechanical aspects and cost than conventional flexible poly(urethane) foams, and, unfortunately, none more thermally sensitive. These polymers, because of their easily pyrolyzed urethane groups and thermally oxidizable aliphatic linkages, exhibit polymer decomposition temperatures of about 250.degree. C. (508.degree. F.), and maximum pyrolysis rates at about 300.degree. C. (598.degree. F.), with a total yield of pyrolysis vapor of about 95%, most of which is very highly combustible. These polymer materials ignite easily with a low power energy source, and when ignited, effect sustained flame propagation even after removal of the heat source. It is standard practice to add fire retardants to urethane foams. These fire retardants can increase resistance to low temperature ignition. However, from thermogravimetric studies, it is evident that the addition of standard fire retardant additives has little or no effect on the maximum decomposition rate, the temperature at which it occurs, or the vapor production yield. In fact, one observes the same average mass injection rates of combustible gases under a sustained radiant heating from flexible poly(urethane) foams whether fire retarded or not.
In addition, even when conventional flame retardants are present, application of a sustained heating rate of approximately 5 W/cm.sup.2 to one poly(urethane) foam seat of a three seat transport array will produce flame spread and ignition to the adjacent seat in less than one minute. This results in sufficient fire growth to permit flames to impinge on the aircraft ceiling in less than two minutes. The time required to produce these events and the resultant increase in cabin air temperature determines the allowable egress times for passengers attempting to escape the aircraft in a postcrash fuel fire. U.S. Pat. No. 4,092,752, issued June 6, 1978 to Dougan and U.S. Pat. No. 3,647,609, issued Mar. 7, 1972 to Cyba have representative disclosures of flame retardants and their use with flammable materials.
Another way to improve seat cushion fire performance is to cover the cushion with a fire blocking layer. These layers can operate in a number of manners. For example, one can use a layer of a less flammable insulative material such as neoprene foam to afford thermal protection. This technique is shown in U.S. Pat. No. 4,060,280, issued Nov. 29, 1977 to Van Loo. Similarly, one can employ a covering which gives a high yield of insulative char when heated. Polyimides and polybenzimidazoles have been among the most widely accepted materials in this role. Heat reflective materials have also been proposed.
Yet another known method to control cushion flammability involves covering cushions with a material that will emit cooling water vapor when heated. This mechanism is commonly referred to as "transpirational cooling". Vonar.RTM., a family of Al(OH).sub.3 power-doped low density and high char yield poly(chloroprene) foams marketed by E.I. Dupont, contains a large fraction of water of hydration, and is one of the best materials in this class currently available. Materials which depend on transpirational cooling for flammability control can be very efficient at high heat fluxes. Their efficiency increases monotonically with the incident heat flux above 7 W/cm.sup.2. However, use of these high performance transpirational blocking layers results in an estimated weight penalty of 1.8 kg per seat. Due to ever-increasing fuel costs, such a penalty can render these blocking layers cost ineffective as fire protection.
Very clearly, there is a pressing need for a cushion construction which affords good fire protection and is cost effective both in terms of weight penalties and intrinsic costs of manufacture and assembly.